Filter constructions used for engines sometimes include a center tube to provide the structural strength necessary to keep the filter media from collapsing due to the pressure differential across the filter as the filter media becomes plugged with contaminant. The inner support tube typically allows for the passage of gas or liquid therethrough, in order for the filter to operate properly. For example, the tube may be perforated or constructed from an expanded metal.
In some constructions, the inner support tube is formed by rolling the metal sheet into a cylinder and either welding the ends together at a seam, or bending the ends together to form a lock seam.